In general, there are two manufacturing techniques employed in the production of silicon nitride articles. These two techniques are hot-pressing and reaction sintering. Hot-pressing of silicon nitride powder yields simple shapes of silicon nitride having a density of 95 to 100% of theoretical. Reaction sintering is a more versatile manufacturing technique. Complex shapes can be formed from silicon particles by such techniques as slip casting and injection molding. For example, in a slip casting technique, silicon particles are initially cast into an article of desired shape. This article is then converted to silicon nitride in a reaction sintering operation in which the article is heated to a high temperature in the presence of nitrogen.
Conventional nitriding techniques require the introduction of the article of silicon particles formed in a shaping operation into an atmosphere containing nitrogen gas. The article is heated to a temperature above a significant reaction temperature at which the silicon and nitrogen can react to form silicon nitride at a measurable rate. This nitriding technique requires porosity in the article of silicon particles so that all of the silicon particles are accessible to the nitrogen gas atmosphere. However, as the nitriding reaction progresses from the surface of the article inwardly, the porosity of the article must be maintained to insure complete nitridation of all the silicon particles.
If portions of the silicon body are not accessible to nitrogen gas, the resultant product is a matrix of silicon nitride with islands of unreacted silicon. This condition usually occurs when conventional nitriding techniques are employed on articles in an attempt to manufacture articles having final densities higher than 2.70 grams/cc. Having free silicon in an article is deleterious to the final silicon nitride product. Having silicon is deleterious in that the thermal shock properties, oxidation resistance and high strength properties are all reduced over that which would be obtainable if all of the silicon was properly nitrided.
It is a principal object of this invention to provide a method of nitriding silicon which is effective in nitriding substantially all of the silicon found in an article.
Those skilled in the art, of course, know that the rate at which silicon reacts with nitrogen gas is temperature dependent. In accordance with this temperature dependency, we have found that a temperature of at least 1800.degree. F. is required as a significant reaction temperature in order to obtain a measurable rate of reaction of silicon and nitrogen. By a measurable rate of reaction we mean that a reasonable amount of silicon reacts with the nitrogen during a reaction period of one hour. For example, if a unit of silicon particles is exposed to nitrogen gas at a temperature above a significant reaction temperature of 1800.degree. F., 5-20% thereof would react during the first hour of exposure.
We have found that the most desirable temperature for the reaction is a temperature in a range from 2400.degree. F. to 2500.degree. F. One does not want to exceed 2600.degree. F. as this is the melting temperature of silicon. If one exceeds 2600.degree. F., the silicon particles have a tendency to melt and coalesce forming large zones of silicon which are impossible to nitride because they are not permeable to the nitrogen. Therefore, in accordance with this specification, we define a significant reaction temperature as being a temperature in the range from 1800.degree. F. to 2600.degree. F., and preferably in the range from 2400.degree. F. to 2500.degree. F.